An oxygen sensor is described, by way of example, in U.S. Pat. No. 5,531,879. In an oxygen sensor of this type, used as a .lambda.-probe, there exists the problem that, when the oxygen sensor heats up, the sensor characteristic curve shifts due to humidity, penetrating exhaust, or residues in or on the housing or mounting parts on the reference side. This phenomenon, known as CSD behavior (Characteristic Shift Down), is treated in the oxygen sensor proposed in the above-referenced U.S. patent by employing for the reference electrode a metal, e.g., gold or a gold-platinum alloy, that is not catalytically active.
In other oxygen sensors of the related art, the CSD behavior is avoided either by the reference side being completely sealed off from the exhaust side, or by an active Pt or Pt/Pd electrode or component parts on the reference side being heated. All of the known measures in the related art for avoiding the CSD problem are either complicated or expensive.